Color filter having vertical color stripes with a nonintegral relationship to CCD photosensors

ABSTRACT

A color encoding filter for use with a discrete photosensor pickup device has vertical stripes. The stripes occur in repeating cycles of color order, with the ratio of the width of a cycle to the width of a photosensor being a non-integer. This results in wider bandwidth in a luminance signal.

BACKGROUND OF THE INVENTION

The present invention relates to color filters, and more particularly,to color filters having vertical stripes for use with cameras having anarray of discrete sensors, such as CCD (charge coupled device) cameras.

As was pointed out in prior patent application Ser. No. 094,285, filedNov. 19, 1979 in the name of R. Rhodes (RCA 74,184), a conventionalvertical stripe color filter when placed in front of the CCD cameralimits the bandwidth of the luminance signal to two-thirds of thatdetermined by the Nyquist limit, which in turn is determined by thenumber of photosensors in a horizontal row of the CCD. In particular, ifthere are 320 photosensitive elements per horizontal line in the CCD,320 sensors are scanned in 53 microseconds for a theoretical bandwidthof about 3 mHz. In the presence of a vertical stripe filter, thebandwidth is actually limited to about 2 mHz for the reasons as setforth in said prior application.

It is therefore desirable to have a color encoding filter for use with aCCD pickup device having a plurality of photosensitive elements whichresults a wider usable frequency response in the resulting luminancesignal.

SUMMARY OF THE INVENTION

In brief, this is achieved by having a filter with a plurality ofvertical stripe color filters arranged in horizontally repeating cyclesof color order, the ratio of the width of a cycle to the width of aphotosensor being a noninteger.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a nonintegrally aligned vertical stripe color filter inaccordance with the invention, together with a portion of a row ofphotosensors of a CCD camera and some signals resulting therefrom;

FIG. 2 shows a decoding system for use with the invention;

FIG. 3 shows the spectral distribution of aliases that result with theinvention; and

FIG. 4 shows another filter in accordance with the invention.

DETAILED DESCRIPTION

FIG. 1A shows the color filter 10 comprising vertically extending colorstripes 12. The stripes 12 occur in triads or groups each including red(R), blue (B), and green (G) color stripes. Eight such triads 1-8 areshown in this figure. The filter 10 may be aligned with a CCD (chargecoupled device) television camera imaging or pickup device. Each CCDdevice consists of a vertically extending array of horizontal rows ofdiscrete image sensors. A portion 14 of the top of one such pickupdevice,as shown in FIG. 1B, comprises a plurality of horizontallyarrayed photosensitive elements 16. Striped filter 10 is shown separatedfrom row 14 of photosensors 16 for clarity, but it should be understoodthat light falling upon any row of photosensors of the CCD passesthrough a portion of filter 10. It will be noted that there are exactlyseven color triads, designated by reference numbers 1 to 7, in thehorizontal length occupied by twelve photosensitive elements 16 in theparticular embodiment of FIG. 1. Each triad of vertically extendingcolor stripes is larger in its horizontal dimension than the width ofone photosensor, but each triad is smaller than the width of twophotosensors. The left side of the left-mostR stripe of triad 1 in FIG.1A is aligned vertically with the left end of photosensor 101 of FIG.1B. However, the right end of the G stripe of triad 1 is not verticallyaligned with the edge of any photosensor of row 14. The next alignmentof the edge of a photosensitive element 16 with an edge of a color triadoccurs at the right edges of the green stripe of triad 7 and the rightedge of photosensitive element 112. The right edge of photosensitiveelement 112 is also aligned with the left edge of a red stripe of triad8. The next alignment, however, does not occur until the right edge ofthe twenty fourth photosensitive element (not shown) in row 14. It canbe seen, therefore, that the alignment of the photosensitive elements ofrow 14 with vertically extending stripe filter 10 has cyclicalvariationsof alignment in the horizontal direction. In a typical CCD camera, eachrow may include 320 photosensitive elements 16. There are, therefore,320 divided by 12, which equals twenty-six and two-thirds repeat cyclesacross a horizontal line.

FIGS. 1C, D and E show the amplitude of the output signal from portion14 of the CCD imager when viewing a flat red, blue or green fieldsrespectively. The output signal is available simultaneously from thephotosensitive elements, but is normally read out sequentially, so thehorizontal dimensions of FIGS. 1C and 1D may be considered to representeither time or the position along row 14 at which the output signal of aparticular photosensor is available. The amplitude of the output signalofa photosensor which occurs when a particular color is viewed isproportional to the area of that particular color filter which isdisposedover that particular photosensor. Note that filter 10 allows acombination of red and blue light to fall on photosensor 101, acombination of blue, green, and red light to fall on photosensor 102,103, etc. The wave forms of FIGS. 1C, D and E are identical except for aphase shift therebetween of one-third (120°) of a repeat cycle (360°) of7 color filter triads to 12 photosensitive elements. Considering now theblue field output signal of FIG. 1D, it will be seen that it includes ahigh frequency component with a period extending over the width of twophotosensors. Therefore, there are 320 divided by 2, which equals 160suchhigh frequency cycles in each horizontal line, and since it takes 53microseconds to scan one horizontal line in an NTSC system, eachhigh-frequency cycle has a duration of 0.331 microseconds and thecorresponding frequency equals 3 mHz. If it were continuous, this 3 mHzsignal would comprise a carrier signal. Note however that at point 18 inthe waveform there is a polarity reversal. This polarity reversal occursevery six photosensor elements, and thus also at points 20 and 22. Thispolarity reversing signal comprises an envelope of the carrier, and ithasa frequency equal to 3 mHz divided by 6 cycles, or 0.5 mHz. Since thecarrier phase reverses periodically, it is suppressed. The result is a 3mHz suppressed-carrier signal with sideband "subcarriers" at 2.5 mHz and3.5 mHz due to beats between the carrier and the envelope signal. Thesamephase reversal occurs in the signal output when viewing a red sceneat points 18a and 22a, and similarly when viewing a green scene atpoints 18band 20b. Therefore, the same double sideband suppressedcarrier signal occurs for red and green scenes with the 120 degree phaseshift noted above. The amplitude of the sidebands contains the colorsaturation information, while their phase contains the hue informationdue to the phase shift between the waveforms as shown in FIGS. 1C, D andE. For example, when viewing a magenta (red and blue) scene, the phasereversal point 18 occurs between photosensors 104 and 105, which is a 60degree phase shift from that of the red scene signal of FIG. 1C.

FIG. 2 shows a decoding system for the above described camera. AC-register24, which may be part of the internal circuitry of the CCDimage pickup device 10, is driven by a 6 mHz clock 26. Register 24provides an output signal which is applied to a low pass filter 58 of aluminance channel andto a bandpass filter (BPF) 28 of a chrominancechannel. Filter 28 has a 2 to 4 mHz bandpass. The output signal from CCDC-register 24 is filtered byfilter 28 and the filtered signal comprisesthe double sideband suppressed carrier signal with sideband subcarriersat 2.5 mHz and 3.5 mHz. The filtered signal is applied to a productdetector demodulator 30 centered at 3 mHz. This demodulator alsoreceives a 3 mHz demodulation signal, which is derived from a divide bytwo frequency divider 32 from a clock 26to supply a carrier fordemodulating the suppressed carrier signal. Demodulator 30 produces asignal having a frequency response extending to 7 mHz due to the sumcomponent of the modulation products. The output of the demodulator 30is passed through a 1 mHz cut-off frequency lowpass filter 34 (for thepurpose of passing the difference components and removing the sumcomponents), and then is supplied to balanced demodulators 36, 38 and 40for color separation. The 3 mHz signal from divider 32 is againfrequency divided by divider 42 by a factor of 6 to produce a 0.5 mHzdrive signal synchronous with the 0.5 mHz sideband signal beingprocessed. This 0.5 mHz drive signal in turn is applied to phaseshifters 44, 46 and 48 to produce 0°, 120° and 240° phase shiftedsignals, which are supplied to demodulators 36, 38 and 40 respectivelyto demodulate along 0°, 120° and 240° axes to separate the colorsignals. The output of demodulators36, 38 and 40, which provide red,blue and green color difference signals respectively, are lowpassfiltered by 0.5 mHz cut-off filters 50, 52 and 54 respectively, and fromthere the lowpassed filtered color difference signals are applied to amatrix 56 of known type.

The output of the C register 24 is also applied to a lowpass filter 58thathas 2.5 mHz cutoff frequency, as mentioned. Its output in turn isapplied to a delay equalizer 60 which compensates for the delay incurredby the color difference signals in going through some of theabove-described circuitry. The luminance signal is then applied to thematrix 56, which then mixes it with the color difference signals toproduce red, blue and green color output signals having a bandwidth from0 to 2.5 MHz.

Other demodulation schemes are possible. For example, one can use threephases of a continuous 2.5 mHz signal derived from clock 26 todemodulate the C register output signal band limited to 2 to 3 mHz by abandpass filter. Rather than demodulating the lower sideband, the uppersideband may be demodulated at 3.5 mHz by restricting the passband ofBPF 28 to 3 to 4 mHz. Further, if in this latter approach, an opticallowpass filter having a 3 mHz cut-off frequency is incorporated in theoptical path including filter 10, high frequency luminance-to-coloraliasing is avoided, since there will be no luminance signals near the3.5 mHz color carrier. It will be noted that since 3.0 mHz is theNyquist limit of a CCDhaving 320 photosensors per horizontal line in anNTSC system, limiting theinput resolution to 3.0 mHz does not restrictthe available luminance bandwidth.

FIG. 3A shows a simplified first order comparison of the aliasing from awide band single color primary color scene into the luminance channel ofan integral aligned vertical stripe color filter as described in theaforementioned Rhodes application when used with a CCD camera."Aliasing" in this context refers to spurious signals developed when adesired signalis processed through a sampling or modulating system. Thesampling occurs due to the presence of the vertical stripes of the colorfilter. It has been assumed that the Nyquist limit of the CCD camera is3 mHz and the amplitude of the output signal at peak white level isnormalized to 1. Line 62 shows the frequency response of an opticallowpass filter having a2 mHz cut-off frequency. This filter is disposedin front of the vertical color stripe filter. A color "carrier" ispresent at 2 mHz when the imageris viewing any scene other than abalanced white scene. It will have an amplitude of 1 unit peak-to-peak.In FIG. 3A the amplitude of this "carrier" has been normalized to 0.75peak-to-peak which provides the same color signal-to-noise ratio as inone version of the non-integral aligned camera of the present invention.As will be seen, curve 64 represents the locus of aliases of a fullstrength primary color signal pickup into the luminance signal. FIG. 3Bshows color "subcarriers" at 2.5and 3.5 mHz resulting from the sidebandsfor the non-integrally aligned case of the present invention. An opticallowpass filter curve with a cut-off frequency of 2.5 mHz is shown bycurve 66. Curve 68 shows the locus of aliases with respect to the 2.5mHz carrier, while curve 70 showsthe locus of aliases with respect tothe 3.5 mHz subcarrier. It can be shown using Fourier analysis of thewaveforms of FIGS. 1C, D, and E that the amplitudes of the subcarriersare not equal. In particular, the amplitude of the 3.5 mHz subcarrier islower than the 2.5 mHz signal. It will be seen that they therefore forma maximum sum of aliases shown by curve 72. For comparison purposes,FIG. 3C shows the curves 64 and 72 on the same graph where it will beseen that the non-integrally aligned case is clearly superior to theconventional integral aligned vertical stripe system with respect toalias amplitude.

With 320 photosensors per line available in a CCD imaging device, theluminance response must be electrically attenuated in the order of 20 to30 DB at 2.5 mHz for the non-integrally aligned case to prevent theappearance of that frequency when viewing a fully saturated color field.This compares with a similar attenuation required at about 2 mHz in thecase of the integrally aligned vertical stripe system. This amounts to a25% improvement, but the improvement can become more significant whenmoreCCD photosensors per line are used. For example, if a CCD wereavailable having 480 horizontal photosensors in each line (Nyquistbandwidth of about 4.5 mHz), and the color signals were frequencyrestricted to 500 kHz, the frequency at which said required attenuationis applied in a conventional aligned vertical striped system could beraised from 2 mHz to3 mHz, while that of the non-integrally alignedsystem could be raised from2.5 to 4 mHz. Under these circumstances, itmight be more advantageous in the non-integral system to limit theluminance response by both optical and electrical lowpass filtering to3.5 mHz. This results in complete removal of high frequencyluminance-to-color aliasing and a considerable reduction incolor-to-luminance aliasing.

The twelve photosensor to seven triad cycle pattern as described aboveis agood choice; however, variations are possible. A twelve to fivepattern works in almost the same manner. The ratio of the photosensor totriad count within a complete cycle is desirably a fractional numbernear two, so that a color triad covers approximately (but not exactly)two photosensors, and the CCD signal output from a color field includesa strong color-conveying "subcarrier" near the CCD imager's Nyquistlimit. If the ratio approaches 3 or over, the color-conveying subcarrierbecomes much lower in frequency and severely compromises the availableluminance bandwidth. If the ratio is below one, each photosensor isassociated with a portion of a complete color triad and color decodingbecomes very difficult. The practical ratio must be fractional since weare limited to numbers between one and three by the above; the onlyother possibility is two itself. However, with a ratio of exactly two,we have two 180° out of phase colors over adjacent photosensors. Thisprovides only a single piece of color information, so that the matrixequations cannot be set up. It is interesting to note that when theratio of photosensors to triads is 1.5, the colors over each pixel addup to either R+B, B+G, or G+R, which is equivalent to a magenta, cyanand yellow filter which is integrally aligned with the photosensor.

A primary-secondary color filter can also be used as shown in FIG. 4,wherecorresponding components have been given corresponding referencenumbers. It will be seen that it is made up of yellow, cyan and greenvertical stripes. This system has the advantage that the color filtercan be made from only two colors, since the green areas can be producedby overlappingthe yellow and cyan areas. These areas must be carefullymatched in transmittance to provide a flat output when viewing a greenfield. One disadvantage of such a primary-secondary color filter is thatthere will be "subcarrier" developed when viewing a white field.However, it is believed that by incorporating a green-clear opticalgrating having a low cut-off frequency, for example near 1 mHz, a numberof advantages can be obtained. First, the high frequency red and bluesignal information is removed by the grating, and many of the mostobjectionable aliases due to these colors will be eliminated. Second,since the high frequencies will be due to green only, a common operatingmethod even in studio cameras, and since there is no "subcarrier" whenviewing green, there will be little aliasing when viewing a widebandgreen or white field. Demodulationof the signals produced by a CCD witha primary-secondary color filter and a green-clear grating can beaccomplished by a demodulator similar to thatof FIG. 2.

What is claimed is:
 1. A filter for use with an image pickup device having a plurality of photosensors, said filter comprising a plurality of vertical stripe color filters arranged in horizontally repeating cycles of color order, the ratio of the width of a cycle to the width of a photosensor being a non-integer.
 2. A filter as claimed in claim 1, wherein said ratio is between one and three.
 3. A filter as claimed in claim 2, wherein said ratio is approximately two.
 4. A filter as claimed in claim 1, wherein said ratio is twelve to five.
 5. A filter as claimed in claim 1, wherein said ratio is twelve to seven.
 6. A filter as claimed in claim 1, wherein each of said cycles comprises a triad of three different colors.
 7. A filter as claimed in claim 6, wherein said colors comprise red, blue, and green.
 8. A filter as claimed in claim 6, wherein said colors comprise yellow, cyan and green.
 9. A filter as claimed in claim 8, said filter comprising a first cyan section and an overlapping second yellow section, said sections producing green at the overlap. 